Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 2. Maintenance, Storage and Repair

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION II - MAINTENANCE, STORAGE, AND REPAIR

3. Inspection.  In order to maintain the highest possible standard of perfection in the accuracy and operation of aircraft instruments, certain checks and inspections are made at regular intervals. General instructions pertaining to these inspections and the accompanying maintenance and repairs are outlined in this section. Specific instructions for individual instruments are included with the detailed discussion of each instrument in succeeding sections.

a. Classification.-The repair, inspection, and maintenance of air-craft instruments may be classified as major repair, base repair, and line maintenance. The scope of the work performed in each of these classifications is definitely outlined in Air Corps Technical Orders and in general is based on the facilities which have been made available to the agencies doing the work. It is important therefore, that all maintenance personnel be informed as to the limits and restrictions placed on instrument repair.

(1) Major repairs.-This class of work is performed at depots only and consists of the application of luminous paint to dials and pointers, replacement of parts, adjustment of internal mechanisms, special lubrication. and all repair work that requires particular skill, special tools, jigs, and calibration equipment. When aircraft instruments are over-hauled or repaired at depots they are insofar as practicable, equal to new instruments both as to accuracy and appearance. Depots prepare suitable log sheets for each instrument tested, identifying the instrument by serial number and showing the points, speeds, temperatures, etc., at which they were calibrated and, in addition, any variations between indicated and calibrated readings. These are retained by the instrument repair unit for possible future reference for a period of at least 2 years. Instruments of a particularly delicate nature are provided with stamped seals when they leave the factory or repair depot after a major overhaul, and service activities are not permitted to break this seal or open the case of these instruments.

(2) Base repair.-This work is performed at stations having base squadrons or detachments which function as the equivalent therof. It consists of bench tests and related minor repair operations that are beyond the scope of line maintenance and includes the reinspection of instruments carried in Air Corps stock at the various stations. The work is accomplished by specially trained personnel who are rated as instrument mechanics. They are familiar with the use and operation of shop and portable field test sets, and in some squadrons, their duties are extended to instrument trouble shooting on airplanes.

(3) Line maintenance.-This is the authorized instrument work performed by the crew chief and his assistants on the airplane. Normally it consists of the daily, preflight. 25-, 50-, and 100-hour inspections, such maintenance as may be required at these inspection intervals, and the removal and replacement of instruments.

b. Daily inspection.-This work is performed by the crew chief on the instruments each day and consists of checking

(1) Pointers for excessive error at zero, except thermometers and absolute pressure-operated instruments, which should show indications consistent with surrounding temperature and pressure conditions.

(2) Instruments for loose or cracked cover glasses.

(3) Instrument lights for operation.

(4) Caging and setting knobs for freedom of movement and correct operation.

c. Preflight inspection.-This work is performed by the crew chief prior to the first flight of the day after the engines are started and consists of checking

(1) Instruments pointers for excessive oscillation.

(2) Readings for consistency with engine requirements and speeds.

d. Fifty-hour inspection.-This work is performed by the crew chief, in some cases with the assistance of an instrument mechanic, and consists of checking

(1) Instruments and dependent units for security of mounting.

(2) Lines and connections for leaks.

(3) Dial markings and pointers for dull and discolored luminous paint.

(4) Operation markings for correctness and discernibility.

(5) Electrical and bonding connections for good contacts and security of attachment.

(6) Vibration absorbers for security of attachment and proper tension.

e. Other inspections.-Other inspections, including 25- and 100-hour inspections, are applicable only to specific items on individual instruments and are discussed where necessary in succeeding sections.

f. Engine change inspection.-Normally this work requires the use of special portable and bench testing equipment and is accomplished by the instrument mechanic with the assistance of the crew chief.

4. Removal and replacement.

a. Instruments are removed and replaced for any of the following reasons:

(1) Failure to indicate.

(2) Inaccurate indication.

(3) Leak in case.

(4) Loose pointer.

(5) Loose or cracked cover glass.

(6) Defective lamp receptacle.

(7) Broken or cracked mounting lug.

(8) Defective setting or caging mechanism.

(9) Defective binding posts or connecting nipples.

(10) Dull or discolored luminous markings.

(11) Any known or suspected defects in internal mechanisms.

b. The removal and replacement of instruments in airplanes require special care and attention. Some of the particular considertions necessary are as follows:

(1) Since these instruments are easily damaged, they should be handled carefully at all times and treated with the same care given an expensive watch.

(2) Unless specifically authorized to the contrary, instruments are always replaced with those of like kind and type.

(3) The location of an instrument or any of its dependent units is not to be changed without proper authority.

(4) When checking or testing an instrument, care is taken not to subject it to undue or abnormal pressures.

(5) If too much force is applied when inserting the mounting screws into the self-locking lug inserts, there is danger of pushing the inserts from the case.

(6) When making connections to the instruments. small tools are to be used and care taken not to twist or put excessive strain on the fittings.

(7) Instrument thread compound is used on all threaded connections.

5. Common repairs.-a. General.-

(1) Common instrument repairs consist of -

(a) Removal and replacement of cover glass.

(b) Removal. balancing, and replacement of pointers (except on electrical instruments).

(2) Repairs of this nature are normally accomplished in the base instrument unit by specially trained personnel.
 

b. Removal of cover glass.-The cover glass is attached to the case of an instrument by means of a snap ring or cover glass hold-down ring. To remove the glass proceed as follows:

(1) When removing a snap ring, care should be exercised to see that the proper type tool is used to prevent scratching the glass or chipping the instrument case. After the snap ring has been removed. the sealing compound should be scraped away with a pointed orange-wood stick. A suction cup may then be used to remove the cover glass.

(2) To remove the cover glass from an instrument which employs a cover glass hold-down ring, it is first necessary to remove the ring screws. Inasmuch as the screws are made of brass and are easily damaged, it is essential that the proper size screw driver is used. If the screw slot has been enlarged to such an extent that a screw driver will not effect removal, it will be necessary to drill the screw and remove it with a screw extracting tool. The seal and seal screw must he removed before any attempt is made to lift the ring from the case. After removal of the ring and gaskets, the cover glass may be removed. Penetrating oil may be used to facilitate removal of corroded screws.

c. Replacement of cover glass. -When replacing a cover glass, the following procedure is used:

(1) The dial of the instrument is dusted with a camel's-hair or soft bristle dial brush.

(2) The pointer is inspected for proper attachment and correct zero setting.

(3) The cover glass is inspected for correct thickness and cleanliness.

(4) With a snap ring installation, after gaskets (if used) are properly installed, the cover glass is replaced, suitable sealing compound applied, and the snap ring inserted. With a hold-down ring installation, the gaskets and lighting ring are properly placed in their respective positions. The cover glass is then replaced in the case. the ring set position, and the screws securely tightened. The seal screw should be resealed.

d. Removal of pointers.-An instrument pointer is lifted from the pointer staff by use of a small tool known as a pointer remover. Lifting of the pointer is accomplished by the tool "feet" which are applied to the under side of the pointer at the place of attachment to the shaft. The pointer of a chronometric tachometer must not be lifted before the pointer screw is removed. Scratching or marring of the instrument dial may he avoided by using heavy paper at those points where the pointer remover touches the dial.

e. Balancing of pointer.-(1) It is essential that the pointer on the modern sensitive aircraft instrument is properly balanced. The check for static balance is accomplished by mounting the pointer on a balancing tool and testing for balance with pointer in various positions. The balancing tool consists of a pin vise and mandrel. The mandrel is inserted into the pointer bushing to support the pointer. The vise is held in a horizontal position, subjected to vibration, and the moving pointer permitted to come to rest. The pointer may be considered balanced if it comes to rest at random positions, whereas an unbalanced condition is indicated when either end consistently assumes a lower position.

(2) After it has been determined which half of the pointer is out of balance, a small amount of solder may be added to or removed from (depending upon condition indicated) the pointer balancing rivet. When adding solder, care should be exercised to protect the luminous paint on the pointer by wrapping a piece of moist cloth around the luminous portion. Minute out-of-balance conditions may he corrected by adding paint (dull flat black lacquer) to the balancing rivet or elsewhere along the hack of the pointer, as determined by the balancing test. The paint on the pointer should be thoroughly dry before performing the balance check.

f. Replacement of pointer.-In some instances the bushing of a friction fit pointer may be found to be oversize. In such case the pointer should be replaced with a new pointer. If it is not possible to obtain a new pointer, and an emergency exists, the bushing of the old pointer is tinned and. if necessary, broached to fit the pointer staff. After the pointer has been replaced the instrument is held in several positions and lightly tapped to insure that the pointer has correct zero setting. The instrument should be operated to cause the pointer to deflect (in the positive direction) ; after the pointer returns to rest. the instrument is vibrated to note if the pointer returns to zero setting. After the setting is satisfactorily accomplished, a touch of paint (dull flat black lacquer) may he applied, if necessary, at the bushing portion of the pointer. The cover glass is replaced as heretofore outlined.

6. Packing, storage, and shipment.-a. Instruments are stored as received in the individual boxes from the manufacturer or instrument repair activities. Unserviceable instruments are likewise packed in individual boxes prior to shipment to depots for repairs, etc. Tissue paper, strips of corrugated paper, and packing felt are used to pack the instruments snugly and prevent their movement within the boxes. Excelsior should never be used for this purpose. Each individual box is securely sealed with gummed paper strips and marked with the inspector's acceptance stamp, imprinted partly on the sealing strips and partly on the box, thus forming a visual means of preventing the removal of the sealing strips without detection. This does not apply to bulky, fragile, or extremely sensitive instruments that require special containers or special shock-absorbing mountings therein such as sextants, octants, chronometers, sensitive gyroscopic instruments, aperiodic compasses, etc., which are stored and shipped in the containers furnished by the manufacturer. Venturi tubes are not packed in the individual boxes with instruments but are packed in a separate container.

b. After packing in individual boxes, a printed form is completely filled out and pasted on each box. The data on this form include the name of the station performing the testing and inspection, date of such testing and inspection, name and type of instrument, manufacturers part number, the Air Corps drawing or specification number, name of the individual performing the inspection, and storage expiration date. This information identifies the contents without opening the box and gives the storage expiration date.

c. The regular air transport supply service is utilized for the shipment of all types of aeronautical instruments whenever feasible. However, the following items are extremely delicate and if air transportation is not available are always shipped by express:

(1) Flight indicators.

(2) Turn indicators.

(3) Navigation watches.

(4) Octants.

(5) Automatic pilot gyro controls.

(6) Precision (type D) altimeters.

7. Reinspection and storage time limit. -a. When in stock, flight indicators, turn indicators, and automatic pilot control units are reinspected at the expiration of each period of 8 months from the date of last inspection: liquid-filled compasses, self-synchronous instruments, and altimeters at the expiration of each period of 1 year; all other instruments are reinspected at the expiration of each 2-year period. If, however, any question of serviceability of instruments in storage exists, more frequent inspections are performed as required.

b. After reinspection, each instrument. if serviceable is properly packed, container sealed, and the printed form filled out and attached. The instrument is then returned to stock for issue. However, if the instrument is found to be unserviceable and cannot be repaired locally, it is forwarded to the nearest repair depot for reconditioning. The limitation of repair by base squadrons is outlined in paragraph 3a(2).

c. Each instrument in a packing box, the sealing strips of which have been broken, is reinspected for serviceability before being returned to stock or issued for service. Each instrument in supply stock is considered unserviceable when the storage limit date as shown on the form secured to the packing box has expired, and is then turned over to the base squadron instrument repair unit for reinspection. Where facilities are not available locally for the work, instruments are returned to the depot for reinspection.